LONG-TERM BEHAVIOR OF THE ORBITAL PERIOD OF ALGOL-TYPE BINARY ST PERSEI

Alternating change of the (O-C) curve for the minima times of ST Persei between 1907 and 1991 is represented almost equally well by (i) a combined effect of two periodic variations, (ii) a periodic variation superimposed on a downward curving parabolic segment, and (iii) a set of line segments. Two photoelectric times of secondary minima invalidate any interpretation of the alternating change in terms of apsidal motion. The component of downward curving parabolic variation requires a secular period decrease of the order of 5.2 10(-7) d yr-1, which is two orders of magnitude larger than the magnetic breaking estimate. Either the Alfven radius of the system is too large (approximately 840 R.) or most of the episodic Roche lobe overflows (approximately 10(-7) M. yr-1) leaves the system. In fact, there is evidence that the downward curving parabolic segment of the (O-C) variation is probably the observed part of a long-period cyclic variation, because the recent period increase is better represented with this assumption. Two periodic variations could well be the light-time effects due to third and fourth component stars in the system. Circular and co-planar orbit assumptions suggest equal masses of about 0.9 M.) for the hypothetical component stars which revolve around the eclipsing pair (with periods of P3 almost-equal-to 22.5 yr and P4 almost-equal-to 83 yr), much beyond the outer Lagrangian points at approximately 10 AU and approximately 27 AU. We introduce a rotostat hypothesis to explain the alternating change in the rotation rate of the cooler component, which is then reflected in a spin-orbit coupling to the orbital period of the system. Incomplete observations of ST Per do not allow any critical test at present. The alternating nature of the period change could be due to either the existence of third and fourth bodies in the system or a magnetically induced rotostat process in the subgiant component star, plus mass and momentum loss from the system.